1. Field of the Invention
The invention relates to the art of metallurgy and foundry production, and particularly to production technology and equipment for out-of-furnace treatment of cast iron (ladle treatment), conducted next to melting the cast iron and designed for upgrading its mechanical properties. The invention may prove most advantageous in producing cast iron having such compact forms of graphite as spheroidal and lamellar.
2. Description of the Prior Art
Due to its high mechanical properties, cast iron having compact forms of graphite finds ever increasing application and production. Those skilled in the art face an important problem which consists in the upgrading of the mechanical properties of cast iron in combination with a decreasing of the expenditures required for its manufacture, and for ensuring high capacity of the production process.
The prerequisite for producing cast iron having spheroidal and lamellar graphite is the deep refining of the cast iron to remove undesirable impurities which would lower its mechanical characteristics, primarily from sulphur and oxygen. It is well known that such a refining may be achieved either by the long-term holding of the iron melt under conditions of high vacuum (of the order of 10.sup.-2 mm Hg) or by introducing various inoculating additives into the melt, e.g. metallic magnesium, cerium, yttrium, calcium, or their mixtures with other elements (master alloys).
The long-term holding of metallic melts, particularly steel and iron, under high vacuum has been known for a comparatively long time (British Patent Specification No. 956,678 and USSR Author's Certificates Nos. 382,695; 440,423; 444,817), though it is applied as a rule only under laboratory conditions which fact is explained by a comparatively high cost of vacuum installations and by a considerable lengthening of the production process.
For this reason, to upgrade the mechanical properties of iron in industrial quantities, it is treated with inoculating additives, mainly magnesium and magnesium-containing master alloys, which are now the most efficient inoculants (USSR Author's Certificates Nos. 500,232; 521,317; 709,690; 749,900).
It should be noted however, that the treatment of iron with metallic magnesium is associated with considerable difficulties since it is accompanied by vigorous "boiling" of the melt, its outbursts from the ladle, bright flashes and abundant gas and fume emissions, all of which deteriorate considerably working conditions for the personnel. Moreover, as magnesium interacts with the ambient oxygen, there takes place intensive, explosion-like burning of magnesium. This results finally in the ineffective consumption of the inoculant, and the formation of in atmospheric pollution.
To reduce the above-mentioned effects encountered in the treatment of iron with metallic magnesium, various devices described in technical literature are used (K. I. Vashchenko, L. Sofroni, Magnievyi chugun, Moscow-Kiev, Mashgiz, 1960, pp. 131-170). However, some of them (ladles provided with massive covers, rotary ladles, ventilated chambers) do not eliminate the contact between inoculating additives and atmospheric oxygen, and abundant emission of gas and fume, thereby resulting in increased consumption of magnesium, while other devices (autoclaves, sealed ladles) though eliminating the above difficulties, are inefficient and hard to operate since they require frequent replacements or readjustments of some assemblies.
It has been also proposed to treat liquid iron by immersing a porous material impregnated with magnesium thereinto, said porous material being particularly coke (French Pat. No. 2,004,076) or a porous refractory material (British Patent Specification No. 1,048,909). However, these methods of treatment have not found wide practical application because of their low efficiency.
Methods of treating cast iron with magnesium-containing master alloys provide for a less intensive process of interaction between the melt and the inoculant than in the application of pure magnesium. For this reason, in application of these methods a simpler equipment in usually employed. Introducing master alloys into the iron melt is carried out in different ways: by immersing billets fixed on bars, into the melt; by feeding a crushed master alloy onto the melt stream while discharging iron from the furnace or pouring iron from one ladle into another; by utilizing sinking pellets supplied to the melt surface (when the density of a master alloy is higher than that of the melt); by charging a master alloy to the bottom of the ladle prior to hot metal charging, etc.
However, in practicing these methods, the contact between inoculating additives and the atmospheric air is also possible, thereby resulting in increased consumption of the additives, formation of considerable amount of slags, and an increase in the amount of undesirable bursts into the atmosphere. Moreover, the application of master alloys in the form of sinking pellets is associated with the use of such expensive and scarce ingredients are copper and nickel.
Also known in the art is a method of intramould treatment of iron (inmould process) wherein inoculating additives utilized as a rule in the form of master alloys are located within a reaction chamber being a part of the gating system of a mould (see Sillen R. Inmold Nodulization with Delayed Pouring in Vertically Ported Molds. Modern Casting, 1979, July, pp. 58-59).
In this method, the process of treating iron proceeds in the course of charging the mould with the melt. Such a production process, though limiting oxidation of the atmospheric oxygen and considerably reducing emission of gases and fumes, has its significant problems which are caused by the fact that the process of inoculation proceeds separately for each casting.
The above consideration results in the necessity of checking each casting from the viewpoint of the degree of spheroidization of graphite in the metal structure. Moreover, with such a method a deep preliminary desulphurization of iron is required, and an additional consumption of iron for filling the reaction chamber with the melt takes place.
Finally, known in the art is a method of out-of-furnace treatment of iron, which combines both above-mentioned kinds of treatment: with inoculating additives and under vacuum conditions (Japanese Patent Specification No. 45-17967). According to this method, magnesium and/or calcium and/or cerium in the metallic form or compounds and mixtures containing at least one of the above elements are added to molten iron as inoculants. Following this, the melt is held under rough vacuum (of the order of 50 mm Hg) for about 5 minutes. Such a combined treatment makes it possible, from one hand, to eliminate utilization of high vacuum, and from the other hand, to lower the consumption of inoculating additives, and at the same time provides for the possibility of preparing iron having fine-grained graphite.
This method can be partially practiced (namely, from the viewpoint of degassing the melt), e.g. by means of an apparatus comprising a receiving tank provided with a discharge opening closed with an insert, a ladle having load trunnions, and a sealable chamber provided with a branch pipe for connecting to a system of air evacuation (USSR Author's Certificate No. 608,839).
The sealable chamber of the above apparatus is a non-detachable shell of a substantially cylindrical form and having a slightly expanded lower portion, disposed vertically and abutting with the upper end face thereof to the lower side of the receiving tank. The lower end face of the shell is hermetically connected to the housing of the ladle for which end the upper portion of the latter is provided with a bearing flange and a sealing gasket. Inside this metallic shell is disposed another shell from a refractory material, converging in the downward direction and entering the neck of the ladle. Thus, on the way of the stream of molten metal moving from the receiving tank towards the ladle is formed an enclosed space defined by the walls of the sealable chamber, in which space a vacuum is established in the process of operation. The ladle is brought under the sealable chamber by means of an industrial truck, and the tightness of abutting the chamber and the ladle is achieved by hydraulic jacks provided in the apparatus.
The above described apparatus operates as follows. In the course of feeding a molten metal into the receiving tank, a hydraulic seal is created within a zone of the discharge opening thereof, thereby providing for the possibility of establishing a required vacuum within the sealed chamber. Following the fusion of the insert closing the above opening, the molten metal goes through the refractory shell where it is subjected to the effect of vacuum, and enter into the ladle.
In spite of the fact that the above method practiced by means of the above described apparatus makes it possible to somewhat upgrade the properties of cast iron, some serious problems arise in practicing this method. Thus, since the introduction of inoculating additives into the molten metal is accomplished prior to performing the vacuum treatment, the process of refining the molten metal from sulphur and oxygen proceeds primarily through the consumption of the inoculating additives. Subsequent degassing the molten metal treated with the additives results only in the intensive evaporation of the inoculating additives since it lowers the boiling temperature of the reagents. The above fact has a particular effect on the removal of magnesium, being the most effective inoculant, from the molten metal, since its boiling temperature is lower than the temperature of the liquid cast iron.
Moreover, degassing for above specified time (5 minutes) is too prolonged since, as investigations have demonstrated, it causes a decrease in the content of the inoculant additives in the molten metal. As a result of the above specified problems, the cast iron produced by such a production process possesses rather low mechanical properties, the consumption of the inoculating additives is rather significant, and the process capacity is relatively low.
It should be also noted that since in the prior art apparatus practicing the above described production process the sealable chamber is made non-detachable and, being a through one, is disposed between the receiving tank and the ladle, the path of the flow of the molten metal from the receiving tank to the ladle is too long, and the molten metal loses a considerable portion of its thermal energy. For this reason, in the cases where the treatment of the molten metal is conducted by frequently repeating cycles and in comparatively small amounts, which is characteristic of machine-building plants, there may occur "freezing" of the molten metal on the surface of the inoculating additives. With such a treatment, the inoculation of the molten metal does not give satisfactory results; moreover, the molten metal will have a relatively low temperature and a poor yield, which substantially impedes its subsequent pouring into the moulds.
It should be also pointed out that the prior art apparatus has a low reliability. This fact is caused by an increased damageability of the ladle flange and of the sealing gasket provided thereon in the course of discharging the treated molten metal through the ladle pouring lip, e.g. when filling the moulds. The above circumstance results in the necessity of restoration of these components of the apparatus prior to conducting next cycles of the non-furnace treatment of the molten metal, which fact practically eliminates the possibility of utilization of this apparatus under conditions where the duration of cycles and breaks between them is only 5-7 minutes.
A low reliability of this apparatus is also caused by the fact that one more sealing gasket is an indespensable component thereof, said gasket being disposed between the receiving tank and the sealable chamber, i.e. within the zone subjected to significant thermal loadings. In a frequent repetition of treatment cycles, such a gasket is also subjected to intensive wear.
Finally it should be noted that the above described apparatus for out-of-furnace treatment of cast iron has a comparatively low capacity and can operate only together with load lifting mechanisms, since it requires a large number of auxiliary crane operations.